Date Published: June 20, 2019
Publisher: Public Library of Science
Author(s): Alice Agarbati, Laura Canonico, Maurizio Ciani, Francesca Comitini, Hernâni Gerós.
Yeast species that colonize the surface of grape berries at harvest time play an important role during the winemaking process. In this study, the use of culturable microbial techniques permitted a quantitative and qualitative inventory of the different yeast species present on the grape berry surfaces of Montepulciano and Verdicchio varieties when treated with conventional and organic fungicides. The results show that the most widespread yeast species at harvest time were Aureobasidium pullulans and Hanseniaspora uvarum, which are considered normal resident species and independent of the grape varieties and treatments applied. Specific differences when comparing the grape varieties were observed in species and were detected at a lower frequency; Pichia spp. were prevalent in Verdicchio, whereas Lachancea thermotolerans and Zygoascus meyerae were found in Montepulciano. In both vineyards, the farming treatments improved the competitiveness of A. pullulans, which was probably due to its reduced susceptibility to treatments that improved the competition toward other fungi. In contrast, the fermenting yeast H. uvarum was negatively affected by fungicide treatments and showed a reduced presence if compared with untreated grapes. Organic treatments directly impacted the occurrence of Issachenkia terricola in Montepulciano grapes and Debaryomyces hansenii and Pichia membranifaciens in Verdicchio. Conversely, a negative effect of organic treatments was found toward Metschnikowia pulcherrima and Starmerella bacillaris. Overall, the data suggest that the yeast community colonizing the grape berry surface was influenced by both grape variety and farming treatments, which characterized the yeast biota of spontaneous must fermentation.
Grapes represent a complex ecological niche where filamentous fungi, yeasts and bacteria cohabitate. The microbiome includes species at a concentration that mainly depends on the grape ripening stage and the availability of nutrients. However, the microbial communities of grapes may be affected by many other variables, such as pedoclimatic factors, viticultural practices, diseases and pests that could modify grape integrity . In general, the yeast populations of mature grapes are comprised by 103 and 105 cells/g, but higher values (approximately one log) have also been found on damaged berries where the availability of sugar and nutrients is higher .
In recent years, the investigation of the geographic distribution of the microbial community of wine grapes revealed a geographic delineation of the yeast communities conditioned by several factors such as cultivar, vintage, climate and agricultural practices [23–26].